Engine brakes or retarders are used to assist and supplement wheel brakes in slowing heavy vehicles, such as tractor-trailers. Engine brakes are desirable because they help alleviate wheel brake overheating. As vehicle design and technology have advanced, the hauling capacity of tractor-trailers has increased, while at the same time rolling resistance and wind resistance have decreased. Thus, there is a need for advanced engine braking systems in today's heavy vehicles.
Problems with existing engine braking systems include high noise levels and a lack of smooth operation at some braking levels resulting from the use of less than all of the engine cylinders in a compression braking scheme. Also, existing systems are not readily adaptable to differing road and vehicle conditions. Still further, existing systems are complex and expensive.
Known engine compression brakes convert an internal combustion engine from a power generating unit into a power consuming air compressor.
U.S. Pat. No. 3,220,392 issued to Cummins on Nov. 30, 1965, discloses an engine braking system in which an exhaust valve located in a cylinder is opened when the piston in the cylinder nears the top dead center (TDC) position on the compression stroke. An actuator includes a master piston, driven by a cam and pushrod, which in turn drives a slave piston to open the exhaust valve during engine braking. The braking that can be accomplished by the Cummins device is limited because the timing and duration of the opening of the exhaust valve is dictated by the geometry of the cam which drives the master piston and hence these parameters cannot be independently controlled.
In conjunction with the increasingly widespread use of electronic controls in engine systems, braking systems have been developed which are electronically controlled by a central engine control unit which optimizes the performance of the braking system. Examples of such systems are disclosed in U.S. Pat. No. 5,012,778 issued to Pitzi on May 7, 1991, U.S. Pat. No. 5,255,650 issued to Faletti et al. on Oct. 26, 1993 (assigned to the assignee of the present application), and U.S. Pat. No. 4,572,114 issued to Sickler on Feb. 25, 1986.
A number of patents disclose the concept of interconnecting a fluid pump with a fluid utilization device in a manner which forms a closed-loop system. Examples of such arrangements in an engine braking system environment are disclosed in U.S. Pat. No. 4,399,787 issued to Cavanagh on Aug. 23, 1983, U.S. Pat. No. 4,592,319 issued to Meistrick on Jun. 3, 1996, U.S. Pat. No. 4,741,307 issued to Meneely on May 3, 1988, and U.S. Pat. No. 4,793,307 issued to Quenneville et al. on Dec. 27, 1988. Examples of such arrangements for use with engine valves generally are disclosed in U.S. Pat. No. 4,765,288 issued to Linder et al. on Aug. 23, 1988 and International Application Publication No. WO 91/03630 published on Mar. 21, 1991.